Meaning of continuous earthquakes in Western Nepal

The knocking of 500 years of accumulated seismic energy.

An earthquake of magnitude 6.4 occurred on Friday at 11:47 pm with its epicenter at Jajarkot in Karnali province of western Nepal.

Hundreds of people have been confirmed dead in Rukum and Jajarkot due to the earthquake. More than 200 were injured. It is said that the number of dead and injured may increase as the rescue operation continues. This is the biggest human and physical damage caused by the earthquake after 2072.

The tremors were felt from Kathmandu to New Delhi, India. Due to the midnight earthquake, people came out of their homes and gathered outside in many places in western Nepal.

Since some time, the remote districts of Jajarkot, Doti, Bajura, Bajhang, Darchula, Achham, Dolpa and other remote districts of western Nepal have been felt continuously. This is knocking the great seismic risk of western Nepal.

In this story, we will talk about what the earthquakes going on in various districts of western Nepal indicate about the seismic risk there.

We all know that Nepal is geographically prone to earthquakes. The constant friction of the Indian and Eurasian plates beneath our surface is generating enormous amounts of power. That power is stored underground. When a large earthquake occurs, the energy stored underground is suddenly released. Then it takes a long time to build up the same amount of power again.

Small earthquakes do not release all the energy stored underground. We cannot even predict when that power will come out. But small earthquakes make us cautious. It indicates that seismic energy is accumulating in that place and that a big earthquake can happen at any time.

To understand this more clearly, let's take the example of the 2072 earthquake.

At that time there was an earthquake of 7.8 magnitude with the epicenter at Barpak in Gorkha. The impact of the earthquake reached from Gorkha to Dolakha via Kathmandu. From this came out the great power that had been stored under the surface of central Nepal for a long time. Geologists say that power has not yet been fully unleashed. Geological studies have shown that part of it is piled up under the surface of Kathmandu. That accumulated power is still causing tremors in Kathmandu.

However, since the 7.8 magnitude earthquake has already occurred, geologists estimate that it may take some time for an earthquake of the same magnitude to occur in Central Nepal including Kathmandu.

Now the question arises, how much power is gathered under the surface of West Nepal?

According to senior seismologist Lokvijaya Adhikari of the National Earthquake Measurement and Research Center, it has been more than 500 years since a major earthquake occurred in Western Nepal. There was an earthquake of magnitude 8.4 or greater in 1505.

According to geologists, the Indian plate must be subducting under the Eurasian plate at a rate of two centimeters per year. In a hundred years that is two meters. That means, 10 meters should have been penetrated in the last five hundred years. However since there has been no earthquake of magnitude 8 in the western Nepal region for 500 years, the Indian plate has not been able to penetrate under the Eurasian plate at its regular speed.

What this indicates is that as much seismic energy is stored beneath the surface of western Nepal as is pushing the Indian plate 10 meters beneath the Eurasian plate. From the geological survey, it appears that a large seismic force has accumulated in this area.

In the year 2072, when an earthquake of 7.8 magnitude occurred, the Indian plate moved about three and a half meters in central Nepal. In this way, geologists have been saying that if 10 meters move at once in West Nepal, it can cause an earthquake of magnitude 9.

How and how much the subsurface rock segments (tectonic plates) move cannot be predicted. In the case of West Nepal, the Indian Plate may or may not move 10 meters at a time.

If it moves 10 meters at once, as geologists say, a magnitude 9 earthquake will cause annihilation, but even if it moves only half, the tremor it will bring will be around magnitude 8. The human and physical damage caused by it can be huge.

We do not know much about the 1505 earthquake in western Nepal. It has not yet been confirmed that the wave of this earthquake spread to Nepal. There is no record of how much damage it caused in Nepal.

The epicenter of the earthquake is considered to be Lower Mustang. Hence it is also called 'Low-Mustang Earthquake'. It is said to have affected a large part of India from New Delhi to North India.

A team including geologist Deepak Chamlagai has been excavating land in various parts of western Nepal for some time to find out where the 500-year-old earthquake affected Nepal. His team also includes Steven Wesnowski, a professor at the University of Nevada, USA, and Yashuhiro Kumahara, an assistant professor at the Hiroshima University in Japan.

The team found some cracks around Dhangadhi and in the upper reaches of Karnali. Looking at its nature, it seems that it could have been 500 years ago, but they have yet to do scientific tests by excavating it.

A long time ago, the American scientist Yule claimed that the crack of the earthquake was found near the Machadi River in Kanchanpur. But its details are not available.

It cannot be said whether earthquake cracks will be found from geological studies. But in West Nepal, the earthquake is constantly knocking. Do we just listen to the knocking or do we prepare for possible risks? What can we do in preparation?

When we asked Chamlagai this question some time ago, he said, "We can assess the impact of earthquakes." If an earthquake of magnitude 8 occurs one day, we can assess the impact of how big the damage would be. Risk mitigation can be prepared after impact assessment.'

After it is known that western Nepal is at risk of earthquake as of today, it is possible to divide the area and measure the ground vibration in the areas where the population is high. This is called the 'micro-tremor' test. It separates which land is at risk in terms of earthquakes," he said.

From this study, it is known how the soil of that place reacts to an earthquake. According to Chamlagai, based on that, rules can be applied to determine the land suitable for building a house and how high a structure can be built.

Sudip Shrestha | Setopati